The production of mechanical pulp is a major industry with over 40 million tonnes of pulp produced annually worldwide. Mechanical pulps are used in a wide variety of papers. Unbleached or slightly bleached pulps are used in the production of newsprint and constitute the largest single usage of mechanical pulps. Mechanical pulps that have been moderately bleached are used to manufacture uncoated products such as supercalendered paper, coated products such as light-weight-coated paper, paperboard and tissue products. Highly bleached mechanical pulps are used in coated and uncoated fine papers such as photocopy paper, technical grades such as carbonless and tissue products. Mechanical pulps are characterized by having high yields in excess of 80% from wood, favorable mechanical properties such as bulk and optical properties such as opacity and lower manufacturing costs than kraft pulps.
The main characteristic of mechanical pulps is that the fibers in the wood chips are separated by mechanical action rather than through chemical action as in kraft pulping. There are several mechanical pulping processes known in the art as taught by Smook, (1992) Handbook for Pulp & Paper Technologists (which is herein incorporated by reference). A minority of mechanical pulp is produced using a stone groundwood method, which consists of grinding debarked logs with a pulp stone to separate the fibers.
The majority of mechanical pulps are made using a refiner method, where wood chips or pulp are passed between plates having raised (bars and dams) and depressed (grooves) segments. The plates are installed in a refiner and at least one of the plates is rotated. The chips or pulp move from the center of the plates to the edges and the chips are converted from chips into coarse pulp or the coarse pulp is further refined by the action of the plates. This process of converting chips to coarse pulp is known as primary refining or defibering and is performed in a primary refiner as is familiar to those skilled in the art. The process of refining the coarse pulp to refined pulp is known as secondary refining and is performed in a secondary refiner as is familiar to those skilled in the art. Other refining stages to further refine the pulp may follow the secondary refining process. The process of defibering, followed by secondary refining and other refining stages, is known as refining.
In the process of the refiner method, the furnish, consisting of softwood or hardwood chips or mixtures thereof, is washed to remove dirt and debris. The chips may then be steamed to remove air and heat the chips prior to refining. The chips may also be pre-treated by compression in a device such as a screw press, followed by introduction to a chemical solution in which the chips relax, absorbing the solution, which process is known as impregnation to those of skill in the art. The chips are then introduced to either an atmospheric or pressurized primary refiner and converted into coarse pulp. The coarse pulp is typically refined in a secondary refiner, after which it may be screened, cleaned or both. Rejects from the screening-cleaning process are re-refined and then added to the main stock. The pulp accepts may be bleached, either reductively and/or oxidatively. The finished pulp may be dried and baled or sent to storage prior to introduction to a paper machine.
One problem that has been facing the industry is the high, and increasing, cost of electricity. The refining of one tonne of mechanical pulp typically requires 800 to 3500 kWh of electricity. For example, at a cost of $0.10/kWh, the cost of electricity is $80 to $350/tonne pulp. This high cost reduces the competitiveness of the pulp in some applications and decreases the profitability of the operation. In addition, the limited amounts of electricity available in some regions can make it difficult for a mill to operate while drawing this much electrical power.
A second problem related to the high electricity usage is the damage to pulp fibers caused by the high energy input. This damage can negatively affect the properties of the final products.
The use of biological products such as fungi, bacteria and enzymes to decrease the amount of chemicals required for processing kraft pulp is known. For example, U.S. Pat. No. 5,591,304 (Tolan et al.) discloses using a hemicellulase on kraft brownstock pulp in the pH range of 7.0 to 9.0 in order to decrease bleach chemical usage.
The use of biological products has been investigated in mechanical pulping. This includes treatments of wood chips or of refined pulp. For example, WO 97/40194 (Eachus and Kaphammer) teaches pre-treating Loblolly pine wood chips with Ceriporiopsis fungi, CLARIANT CARTAZYME® HS enzyme (contains xylanase) or mixtures of CLARIANT CARTAZYME® NS enzyme (contains xylanase) and SIGMA® lipase enzyme for long periods of time, which are not practical in a mill. For example, the fungal treatments are for 8 to 14 days and the enzyme treatments (e.g. CLARIANT CARTAZYME® HS or CLARIANT CARTAZYME® NS enzyme and SIGMA® lipase) are for 48 hours. Furthermore, these long fungal treatment times are not suitable in cold or warm climates due to the extremes of temperature in these climates. The enzyme treatment (CLARIANT CARTAZYME® HS) had no effect on refiner energy when the enzyme was added by submerging the wood chips in a buffered solution, but slightly decreased the refiner energy by 100 kWh/t when the enzyme was added using an IMPRESSAFINER® (a chip impregnation device). Some of the benefits desired by the industry were obtained in this method (e.g. improved pulp properties); however, there were no significant reductions in refiner energy use and the lengths of the treatment periods are impractical.
WO 2004/022842 A1 (Peng et al) teaches treating wood chips with a pectinase prior to primary refining of the chips. Energy savings of up to 500 kWh/t are obtained compared to an untreated control. This treatment can be performed in the presence of a chelant (diethylenetriaminepentaacetic acid) or sulfite, but no additional energy reductions above that provided by pectinase treatment in the absence of the chelant are observed. Due to the expense of pectinase, such a treatment would not be practical in a mill setting.
Viikari et al. (Pretreatments of Wood Chips in Pulp Processing, in Paavilainen, L. ed., Final report—Finnish Forest Cluster Research Programme, WOOD WISDOM, 1998-2001, Report 3, pp. 115-121; incorporated herein by reference) discuss pretreating Norway spruce softwood chips with fungi or enzymes prior to refining. The fungal treated chips required 15% less refining energy to produce a pulp of a given freeness and having an improved tensile strength but lower brightness. The energy consumption for refining was decreased by using enzymes that modify lignin and by 10-20% when using enzymes that modify cellulose or hemicellulose. No details of the methods, conditions of pretreatment or the enzymes used are provided.
The treatment of pulp after primary refining to decrease energy requirements has also been investigated. U.S. Pat. No. 6,267,841 (Burton) teaches treatment of primary refiner hardwood or softwood pulp with enzyme to decrease the energy requirements of the secondary refining operation. EP 0 687 320 B1 and EP 0 692 043 B1 (Viikari et al.) disclose treating once refined pulp with cellulase or a cellulase/mannanase mixture prior to secondary refining in order to decrease the refining energy. One disadvantage of treating pulp after primary refining is that most of the refining energy is consumed in primary refining, so treating pulp after primary refining can only have limited impact. Another disadvantage is that most mills transfer the pulp directly from the primary to the secondary refiner and there is no equipment or storage tank provided to treat the pulp between the two refining stages. An additional storage tank would be required to implement this technology.
EP 0 430 915 A1 (Vaheri '915) teaches the use of hydrolytic enzymes, from either Aspergillus or Trichoderma fungi to decrease refining energy. The enzymes may be mixed with either wood, wood chips or pulp refined at least once prior to subsequent refining. An example involving xylanase treatment of defibered spruce (once refined) pulp, at 20° C., for a 3 hour period is provided. An energy savings of about 300 kWh/tonne was obtained. However, the specified conditions are not practical for use in a mill setting.
WO 91/11552 (Vaheri '552) discloses a method of treating fibrous material, including wood chips and pulp, simultaneously with hydrolytic and oxidizing enzymes and adjusting the redox potential to 200 mV prior to primary or secondary refining and a corresponding reduction in the refining energy. However, the oxidizing enzymes described by Vaheri '552 (WO 91/11552) are not commercially available and adjusting the redox potential is costly.
Therefore, in spite of previous efforts, there is no commercially viable means of using biological products or methods for reducing refining energy. There remains a need in the art for novel products that will decrease refining energies, lead to improved fiber properties and be commercially viable.